1. Field of the Invention
The present invention generally relates to the black (or dark) level calibration (BLC) for an image sensor, and more particularly to an analog dark-signal averaging (dark average) circuit and method for the image sensor.
2. Description of Related Art
Semiconductor based image sensors such as charge-coupled devices (CCDs) or complementary metal-oxide-semiconductor (CMOS) sensors are widely used, for example, in cameras or camcorders, to convert images of visible light into electronic signals that can then be stored, transmitted or displayed.
Due to the imperfectness of electronic circuitry, leakage current (or dark signal) exists even when no light is received by the image sensor. This unwanted dark signal is accumulated along with a desired data signal, and, for the worse, the dark signal is indistinguishable from the data signal. The accumulated dark signal consumes image dynamic range and reduces image contrast, and thus degrades image quality. In order to suppress or correct the dark signal, a black (or dark) level calibration (BLC) is thus needed. In the BLC operation, dark signals of one or more light-shielded pixels are collected as a black or dark level reference, which is then subtracted from the integrated signal, therefore improving the image quality.
Digital domain BLC is one of the conventional methods used to calibrate the black or dark level in an image sensor. FIG. 1 shows a conventional digital BLC system. Specifically, light-shielded pixels are amplified by a programmable gain amplifier (PGA) 11 and then converted into digital equivalents by an analog-to-digital converter (ADC) 12. The digital outputs of the ADC 12 are averaged by a digital circuit, such as a digital signal processor (DSP) 14. The averaged output is then converted back to an analog equivalent by a digital-to-analog converter (DAC) 16, and is then used to compensate the PGA 11. The conventional digital BLC system needs to obtain the statistical value of the dark level before feeding back the dark level to perform the calibration. Due to the nature of the dark level statistics in the digital domain, a lot of time is involved in obtaining the statistical dark level. Therefore, the number of the frames per second is greatly limited, and the video rate thus falls. Moreover, the performance of the digital BLC becomes worse when a fault pixel or pixels, such as hot pixels, exist in the image sensor.
For the reason that conventional BLC methods, particularly the digital domain BLC, cannot quickly calibrate the dark level for the image sensor, a need has thus arisen to propose a novel scheme to quickly and effectively obtain the dark level for performing the BLC.